A terahertz (THz) wave indicates an electromagnetic wave having unique properties in the range of 0.3˜30 THz, which shows sensitivity for the optical phonon mode related with molecular rotation and vibration, high absorbability for semiconductor and conductor materials, low absorbability for non-conductive materials, and harmlessness for biological systems. Accordingly, THz wave application fields such as THz image systems and THz time-domain spectroscopy based on such a THz wave having unique properties are regarded as more and more important.
However, a THz wave generation source (light source) reported to date exhibits low efficiency and narrow bandwidth, and thus THz wave application techniques are limited.
The most important aim of the THz wave techniques is to develop a THz generation source having a wide bandwidth and high efficiency.
Electro-optic crystals are a promising material for generating a wide THz wave by use of a method such as optical rectification (OR) or difference frequency generation (DFG).
Currently useful inorganic crystals such as ZnTe and GaAs are THz light sources widely employed for these techniques.
Recently, by using organic electro-optic crystals, great improvements in THz generation and detection have been made. Organic crystals having the greatest performance based on dimethylamino-stilbazolium and non-ionic configurationally locked polyene (CLP) groups show large macroscopic nonlinearity with a maximum electro-optic coefficient of riii=53 pm/v at 1.3 μm, whereas inorganic ZnTe crystals show r41=4.1 pm/v.
So large macroscopic nonlinearity of the organic stilbazolium and CLP crystals compared to the inorganic crystals manifests greater THz generation efficiency.
Currently, organic electro-optic crystals having high nonlinear optical properties have to improve or optimize not only nonlinear optical properties but also various crystal properties to achieve high THz wave generation efficiency.
For example, OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) and DAST (N,N-dimethylamino-N′-methylstilbazolium 4-methylbenzenesulfonate) crystals having the greatest performance exhibit the highest THz generation efficiency at a wavelength in the infrared (IR) range of 1.0˜1.6 μm.
Under such a pump wavelength range, that is, in the near infrared (NIR) range of 0.7˜1.0 μm corresponding to an important wavelength range for a commercially available high-power femtosecond Ti:sapphire laser supply source, these crystals show low THz wave generation efficiency by a phase matching condition.
Furthermore, high photochemical stability and a large operating wavelength range are regarded as important, which are related with linear absorption properties of crystals. Therefore, development of organic crystals for THz generation with high efficiency in the NIR range is also important.